GaN-based transistors are increasingly used in power devices. AlGaN/GaN-based lateral field-effect transistors, wherein polarization charges at the heterointerface produce a high-density, high-mobility two-dimensional electron gas (2DEG) and thus effectively reduce the on-state resistance, are predominantly used. Lateral GaN transistors can be fabricated on low-cost, large-diameter Si substrates. However, the threshold voltage of most lateral GaN transistors is not high enough for use in high-power applications such as automotive applications, where a threshold voltage above 3-5 V is preferred in order to prevent false operation caused by noise. Furthermore, in these transistors increasing the breakdown voltage is achieved by increasing the gate-drain spacing, which reduces the effective current density and increases the chip size and cost for a required amperage rating.
Alternatively, vertical GaN devices on free-standing GaN substrates have been attracting attention. In vertical devices the breakdown voltage is increased by increasing the thickness of the drift region without sacrificing the device size, so that high-power density chips can be realized. The paper “Vertical GaN-based trench metal oxide semiconductor field-effect transistors on a free-standing GaN substrate with blocking voltage of 1.6 kV”, by Tohru Oka, Yukihisa Ueno, Tsutomu Ina, and Kazuya Hasegawa (Applied Physics Express 7, 021002 (2014)), discloses vertical GaN-based trench metal-oxide-semiconductor field-effect transistors on a free-standing GaN substrate having a blocking voltage of 1.6 kV.
FIG. 1 illustrates schematically a prior art Vertical GaN-based trench MOSFET 10 on a free-standing GaN substrate 12. Substrate 12 is a strongly N doped GaN substrate 12, on which is formed a GaN epitaxial layer 14. A lightly N-doped drift region 16 is formed in the bottom of layer 14, on top of substrate 12. According to the present disclosure, lightly doped can mean having a doping lower than 1E18 cm−3. A strongly P-doped base layer 18 is formed in layer 14 on top of drift region 16, and a strongly N-doped source region 20 is formed on top of base layer 18. According to the present disclosure, strongly doped can mean having a doping higher than 1E18 cm−3. A source contact 22 can be formed on source region 20. A gate trench 24 having at least one vertical wall 26 extending along a portion of source region 20 and a portion of base layer 18, has a bottom wall 28 in contact with the drift region 16. An insulating layer 30 covers the inside of trench 24. A gate region 32 can be formed on top of insulating layer 30. A gate contact 34 can be formed on gate region 32. A drain contact 35 can be formed on the bottom of substrate 12.
The P-doping of base layer 18 can be accomplished by incorporating a P-dopant, such as magnesium, in epitaxial layer 14. It has been noted however that P-type doping in GaN is usually inefficient. This is because: 1. magnesium dopants in GaN are largely passivated by hydrogen atoms; 2. magnesium doping has a high ionization energy. Insufficient P-type doping in GaN leads to reduced performances of the transistor 10, such as low threshold voltage and high base resistance.